[efi] Allow currently executing image to be opened via virtual filesystem

[efi] Allow currently executing image to be opened via virtual filesystem

When invoking a kernel via the UEFI shim, the kernel image must be
accessible via EFI_SIMPLE_FILE_SYSTEM_PROTOCOL but must not be present
in the magic initrd constructed from all registered images.

Re-register a currently executing EFI image and mark it as hidden,
thereby allowing it to be accessed via the virtual filesystem exposed
via EFI_SIMPLE_FILE_SYSTEM_PROTOCOL without appearing in the magic
initrd contents.

Signed-off-by: Michael Brown <mcb30@ipxe.org>